U.S. officials say an assault that killed the U.S. ambassador to Libya and three other Americans in the Libyan city of Benghazi may have been planned rather than being a spontaneous outburst of anti-American sentiment.

The officials said Tuesday's attack on the U.S. consulate in Benghazi by a mob armed with rocket-propelled grenades appeared to be part of a coordinated and complex operation. But, other U.S. officials said it was too early to determine the identities of the perpetrators of the assault or whether they timed it to coincide with the 11th anniversary of the September 11 terrorist attacks on the United States.

Libyan officials said there were two distinct assaults that killed U.S. Ambassador J. Christopher Stevens and three colleagues in Benghazi. In the first, they said a mob stormed the U.S. consulate and torched the building, prompting some Americans to evacuate to a nearby safe house. The Libyan officials said the safe house came under rocket fire hours later. There was no immediate confirmation of that account from U.S. authorities.

Witnesses said Libyan security personnel tried to repel the assault on the consulate but were overwhelmed. Initial reports said the mob was angered about an allegedly U.S.-made amateur film that mocks the Prophet Muhammad. Hours earlier, an Egyptian mob angered by the film stormed the U.S. embassy in Cairo and tore down the American flag.

U.S. President Barack Obama condemned the killing of the four Americans and vowed to work with the Libyan government to bring the attackers to justice.

He also said the United States “reject(s) all efforts to denigrate the religious beliefs of others,” – a reference to the controversial film.

U.S. Joint Chiefs of Staff Chairman General Martin Dempsey made an unusual phone call to an anti-Muslim American pastor and urged him to “consider withdrawing” support for the film. Pastor Terry Jones has presided over a burning of the Quran, an act that triggered Muslim outrage. U.S. military officials were concerned that any promotion of the film by Jones could provoke violent attacks on U.S. forces in Afghanistan.

A trailer for the film about the Prophet Muhammad was posted on YouTube in July. An Arabic-language translation of the video began circulating in the Middle East in recent days.

Speaking Wednesday at the White House Rose Garden with U.S. Secretary of State Hillary Clinton by his side, Mr. Obama said he has ordered increased security at U.S. diplomatic posts around the world. Most American diplomats in Libya were being evacuated. After his address, Mr. Obama went to the State Department to console diplomatic staff.

Ambassador Stevens is the first U.S. envoy to be killed on duty in more than 30 years. He was a career foreign service officer and one of the most experienced American diplomats in the region. He had taken up his post in the Libyan capital, Tripoli, in May. It was unclear why he was in Benghazi at the time of the assault.

Stevens was widely admired by Libyan rebels for his support of their uprising that overthrew longtime Libyan dictator Moammar Gadhafi last year. Secretary Clinton said Stevens “risked his own life to lend the Libyan people a helping hand to build the foundation for a new, free nation.” She also said the relationship between the United States and Libya will not be “another casualty” of the attack.

U.S. Foreign Service Information Management Officer Sean Smith also was killed in the Benghazi assault. The names of the other two American fatalities have not been released.